1. Field of the Invention
The present invention relates to a method, an apparatus and a burner for fabricating an optical fiber preform in an efficient and stable manner at a high fabrication rate.
2. Description of the Prior Art
Optical fiber preforms are fabricated by the MCVD (Modified Chemical Vapor Deposition) method, the OVD (Outside Vapor Deposition) method or the VAD (Vapor-phase Axial Deposition) method. One of the most important problems in the production of optical fibers is how to produce high quality optical fibers in an economical way by these processes; that is, how to produce a large amount of optical fiber during a short period of time so that their cost can be reduced. It has been expected that this problem can be solved especially by a process capable of fabricating large optical fiber preforms at a higher rate.
In order to attain a high synthesizing rate in the VAD process, fabrication process using plural burners in a multi-stage and an improvement in burner have been considered. In the case of an optical fiber preform fabrication process using plural burners in a multi-stage, plural burners are disposed around the area where a porous preform is grown. Therefore, the synthesizing rate can be increased with the increase in the number of burners. However, when compared with an optical fiber preform fabrication process using only one burner, the preform fabrication process using plural burners in a multi-stage is inferior in characteristics, stability and reproducibility, because of the interference between the flames from the plural burners.
In the case of an optical fiber preform fabrication process with only one burner, raw materials for glass must be supplied in a larger amount in order to synthesize a porous preform at a high rate, but when the supply of the raw materials for glass is increased, an amount of the raw materials which have not reacted is also increased and consequently the flame stream is disturbed. As a result, there arises the problem that the yield is reduced.
Meanwhile, in order to fabricate a porous preform of a large diameter, there has been proposed a process in which a flow rate is taken into consideration so as to optimize the fabrication conditions as disclosed in "Fine Glass Particle-Deposition Mechanism in the VAD Process", by H. Suda et al., Fiber and Integrated Optics, Vol., 4, No. 4, pp. 427-437. However, a yield obtained by this process is too low to be employed as a process for fabricating porous preforms.
In the case of the fabrication of large optical fiber preforms at a high rate by the VAD process, an amount of the supply of raw materials for glass must be increased. Therefore, in order to improve the reaction efficiency of glass raw materials, there has been proposed a method in which a multi-flame consisting of a plurality of flames surrounded by each other is used. For instance, Japanese Patent Application No. 219,380/1983 which was filed Nov. 24, 1983 by the same inventors and which has not been laid-open and accordingly does not constitute prior art discloses "A burner for use in synthesis of fine glass particles" in which the effective flame length is increased by using double-flames consisting of an inner flame and an outer flame surrounding the inner flame and by spacing the inner flame rearwardly of the outer flame so as to control the size of fine glass particles.
FIG. 1 shows a construction of a double-flame burner as one example of such a multi-flame burner of the type described above. In FIG. 1, reference numeral 1 denotes an inlet for supplying glass raw materials which are to be charged into an inner flame 8; 2, an inlet disposed around the raw material supply inlet 1 for supplying a combustible gas for the inner flame 8; 3, an inlet disposed around the combustible gas supply inlet 2 for supplying glass raw materials which are to be charged into the outer flame 9; and 4, an inlet disposed around the raw material supply inlet 3 for supplying a combustible gas for the outer flame 9. Reference numeral 5 denotes an orifice for the inner flame; and 6, an orifice for the outer flame. The orifices 5 and 6 are independent of each other. Reference numeral 7 denotes a layer of raw materials reacting in the inner flame 8; 10, produced fine glass particles; and 11, a porous preform which is growing. a represents the length of the inner flame 8; and b, the length of the double flame consisting of the inner and outer flames 8 and 9. The inner flame orifice 5 is retractably disposed with respect to the outer flame orifice 6 so that the inner flame 8 can be spaced rearwardly of the outer flame 9 by a distance l. The distance l can be adjusted in response to an amount of the supply of glass raw materials.
In the case of the double-flame fabrication process, the increase in flame length due to the outer flame results in the increase in an amount of the fine glass particles deposited. In other words, when the double-flame fabrication process is employed, a deposition rate of fine glass particles is increased. Especially, the greater an amount of a glass raw material, the more remarkable the advantageous effect of the double-flame fabrication process becomes.
A possible explanation for such an effect follows. The increase in flame length accelerates the decomposition of a glass raw material so that a passing time period during which the fine glass particle passes through the flame region is increased and consequently the fine glass particle synthesized is increased in size.
FIG. 2 shows the relationship between a passing time during which the fine glass particle passes through the flame region and a specific surface area of the fine glass particles when the double-flame fabrication process is employed. FIG. 2 clearly shows that the longer the passsing time during which the fine glass particle passes through the flame region, the smaller the specific surface area of the fine glass particles becomes and the larger the particle size of the fine glass particle becomes. It follows, therefore, that when the flame length is increased, the passing time during which the fine glass particle passes through the flame is increased and consequently the size of the fine glass particle is enlarged.
In summary, when a double-flame burner is used and the inner orifice thereof is spaced rearwardly of the outer orifice, it is expected that the size of the fine glass particle can be increased and consequently the deposition rate of fine glass particles is also increased. Thus, optical fiber preforms can be synthesized at a high rate.
The transmission state of light through an optical fiber is dependent on a diameter of a core, the difference in refractive indexes between a core and a cladding, a refractive index profile and so on. Various combinations of these factors provide optical fibers with a variety of desired properties and characteristics. So far, in order to control the diameter of a core, the diameter of a preform is controlled. A difference in refractive indexes and a desired refractive index profile can be controlled by controlling a concentration distribution of a dopant which is added to the glass raw materials to control a refractive index. Typical dopants are B.sub.2 O.sub.3, GeO.sub.2, TiO.sub.2, P.sub.2 O.sub.5 and the like. For instance, see "Refractive-Index Profile Control Techniques in the Vapor-Phase Axial Deposition Method" by Sudo et al., The Transactions of the IECE of Japan, Vol. E64, No. 8, August 1981. It is known that the concentration of the dopant GeO.sub.2 is dependent on the temperature distribution over a growing surface of a preform which is being synthesized.
FIG. 3 shows the relationship between a surface temperature of a porous preform deposited when the mixture of SiCl.sub.4, which is a raw material for SiO.sub.2, and an additive of GeCl.sub.4 are introduced into a burner and a GeO.sub.2 concentration. When the surface temperature is maintained in a range between 500.degree. and 800.degree. C., the noncrystalline form GeO.sub.2 which is effective in determining a desired difference in refractive indexes is added in proportion to a temperature (See THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN Vol. J65-C, No. 4, 1982, pp. 292-299). The above-described relation is utilized in a conventional VAD process in such a way that a desired surface temperature distribution of a deposition region of a porous preform being synthesized is maintained by controlling the position relationship of a porous preform being synthesized with respect to a synthesizing burner into which a glass raw material and additive raw material are introduced and by controlling amounts of a combustible gas and a burn-supporting gas introduced into the burner so that a difference in refractive indexes and a desire refractive index profile are controlled to synthesize an optical fiber preform (Japanese Patent Application No. 75,934/1980 or U.S. Pat. No. 4,367,085 which corresponds to the Japanese Patent Application No. 75,934/1980). In case of a multi-flame burner, however, a technique of controlling a refractive index profile has not been established.
As described above, when a multi-flame burner is used, the effect of controlling the size of fine glass particles has be confirmed, but the improvement of the reaction efficiency of a glass raw material has not been satisfactory. Moreover, in the case of the actual fabrication of preforms, there arise the problems that the growth of a preform is not stable (resulting in cracks in porous preforms and disturbances in the growing surfaces of porous preforms) due to an non-uniform flame temperature distribution, and that a refractive index profile cannot be controlled. Under some conditions for supplying various gases in the fabrication of porous preforms, the growth of porous preforms becomes extremely slow or becomes non-stable. Therefore, it has been very important to throughly comprehend the conditions under which preforms can be fabricated with a high degree of reproducibility so that a fundamental fabrication process can be established.
Furthermore, an optical fiber which is obtained by drawing an optical fiber preform comprises at least a core through which light is transmitted and a cladding which surrounds the core. Therefore, in the case of the fabrication of a preform, fine glass particles with different composition types are deposited in such a way that the structure of a preform thus obtained is substantially similar to that of an optical fiber with desired structure and characteristics. In order to fabricate optical fiber preforms with a fundamenal structure consisting of a core and a cladding, the following two processes are generally employed.
One process is called a partial synthesizing process in which a preform containing a portion corresponding to the core of an optical fiber or the core and a part of the cladding is synthesized by the VAD process. The core portion is consolidated into transparent glass, and then the preform is inserted into a quartz tube as a cladding to form integrally a preform. According to this process, only a core and a portion surrounding the core through which light transmits are formed by the deposition of fine glass particles which takes a long time and an outer cladding which has less influence on the transmission of light through an optical fiber is made of a quartz tube. Even though a quartz tube used in this process has less influence upon the transmission of light, the quartz tube has some influence on the mechanical strength of an optical fiber drawn. Therefore, it is necessary to use a quartz tube with a high degree of purity. As a result, there arise the problems that two steps are required and that the fabrication cost is expensive.
The other process is called an all synthesizing process in which fine glass particles for forming a core and fine glass particles for forming a cladding are so deposited that a porous preform with a desired cladding-to-core-diameter ratio can be obtained. In general, a cladding must be made thick. For this reason, burners for synthesizing a cladding must be provided in a plurality of stages.
Table 1 below shows the fabrication processes, required cladding-to-core-diameter ratios and required weight ratios of typical optical fibers used in communication systems at present.
TABLE 1 ______________________________________ Synthesized Core cladding weight of weight of core cladding Types diam- glass diam- glass optical Fabrication eter (relative eter (relative fiber Process (.mu.m) value) (.mu.m) value) ______________________________________ graded all synthesis 50 1 125 5.25 index type fibers large core all synthesis 80 1 150 2.51 fibers partial 10 1 50 24 synthesis of cladding single mode all synthesis 10 1 125 155.25 fiber ______________________________________
When a conventional burner is used, fine glass particles can be synthesized at a maxium rate of 1.7 g/min and many burners must be arranged in a plurality of stages in order especially to form the cladding of a preform as shown in Table 1. The above-described process has the problems that it takes a long time before a preform which is being synthesized is brought into steady state, that the length of a preform becomes long before the preform is brought into steady state, and that it is impossible to fabricate a preform in a stable manner for a long time period due to the interference among the plural burners. These problems arise typically in the case of the fabrication of preforms for single mode optical fiber as shown in Table 1. Therefore, in order to synthesize a preform for single mode optical fiber, cladding burners must be disposed in 3-5 stages and a diameter of a preform for a core must be made small. This is the reason why some of the single mode optical fibers shown in Table 1 are fabricated by the partial synthesis of cladding, which corresponds to the former process.
In the fabrication of optical fibers in various types, it is difficult to attain a high manufacturability merely by arranging synthesizing burners with the same performance in a plurality of stages. Therefore, the studies on the burners having different performance have been made.
In the case of the fabrication of preforms for single mode optical fibers by the all synthesis process, in order to increase a cladding-to-core-diameter ratio, there has been proposed a process in which fine glass particles are deposited in such a way that a diameter of a preform for a core is made as small as possible. However, it is known that even when the diameter of a burner is reduced or an amount of a raw material supplied to a burner is decreased, the diameter of a porous preform obtained cannot be reduced less than 20 mm. In order to deposit porous preforms with a diameter less than 20 mm, Japanese Patent Application Nos. 129,530/1979 and 93,841/1980 or corresponding U.S. Pat. No. 4,345,928 proposed the use of an eccentric burner in which a raw material supply port is disposed eccentrically in a cross section of the burner. The eccentric burner is used in such a way that the eccentric raw material supply inlet is directed downward. As a result, the fine glass particles which are being synthesized in the flame can be prevented from spreading in the lateral direction so that a preform with a diameter less than 20 mm can be synthesized.
Another method for obtaining a high cladding-to-core-diameter ratio is to enlarge a cladding system. In this method, however, the same problems as described above arise when a large number of burners are used. Therefore, it has been desired to invent a fabrication method with a high performance burner which has a deposition rate per burner is increased.
Furthermore, when porous preforms become large in size by increasing a synthesis rate, the following problems arise when they are consolidated.
A large poous preform was placed in an electric furnace and consolidated under the same conditions as employed when porous preforms fabricated by the conventional VAD process are subjected to consolidation. The porous preforms fabricated by the conventional VAD process became transparent, but the large preform fabricated by the above-described high-rate synthesizing process did not become transparent or remained a little white color. Therefore, it becomes clear that the large preforms fabricated by the high-rate synthesizing process cannot be made transparent. The porous preforms fabricated by the conventional VAD process and the preforms fabricated by the high-rate synthesizing process are different in (1) that the diameter of the fine particles of the former is less than 0.1 .mu.m while the diameter of the fine particles of the latter is of the order of 0.2 .mu.m, (2) that the bulk density of the former is about 0.23 g/cm.sup.3 while the bulk density of the latter is about 0.39 g/cm.sup.3 and (3) that the size of the former is about 60 mm in diameter while the size of the latter is about 130 mm in diameter. It is, therefore, considered that the porous preforms fabricated by the high-rate synthesizing process must be consolidated into a transparent body under some special conditions.
FIG. 4 shows a conventional apparatus for fabricating optical fiber preforms. Reference numeral 101 denotes a seed rod made of quartz glass; 102, a consolidated preform; 103, a porous preform; 104, a muffle made of quartz glass; 105, an electric furnace for consolidation; 106, a reaction vessel made of Pyrex glass; 107, a burner made of quartz glass; 108, a glass material supply system; 109, a gas seal; and 110, a pressure gauge for measuring a pressure inside the electric furnace 105. The muffle 104 has flanges at positions indicated by A and B so that the gas-tightness can be maintained.
When the porous preform 103 has a diameter of the order of 60 mm, the electric furnace 105 must be generally heated to 1500.degree. C. in the consolidation step. Furthermore, in order to dehydrate the porous preform 103, a chlorine series dehydrating agent such as thionyl chloride, chlorine gas or the like must be used. As a result, in view of sealability and fabrication capability, there is no way but to use the muffle 104 made of quartz glass. However, a temperature at which the muffle 104 can be used without being softened is up to 1300.degree. C. When the muffle 104 is used at 1500.degree. C. without causing any deformation, an inner pressure in the inside of the electric furnace 105 must be monitored by means of the pressure gauge 110. An optimum value of the inner pressure varies in response to the pressure in the muffle 104 and it is difficult to control such an optimum value. Moreover, the muffle 104 is often used at a temperature higher than a maximum allowable temperature, it is likely to become opaque and cracks propagate in it. As a result, the muffle 104 is soon used up.
When the diameter of a porous preform is increased, the thermal absorption of the porous preform is increased and the inner diameter of the electric furnace is enlarged, so that thermal losses are increased and the load density of a heater becomes high. In the case of an electric furnace in the form of a tube or pipe, it is difficult to change an area of the heat radiating surface over a wide range, so that the surface temperature of the heater is increased with the increase in load density. For instance, when a carbon resistance furnace with an inner diameter of 150 mm is used and a muffle made of quartz glass is inserted in the furnace in order to consolidate a porous preform having an outer diameter of 130 mm, the temperature of a heat source must be raised at a rate of 3.degree. C./min to 1550.degree. C.
Therefore, in the case of an electric furnace which has a suitable inner diameter and is capable of increasing the temperature to a point at which transparent glass can be obtained, energy losses are high and a temperature of a heat source must be set at a high temperature. As a result, the power consumption of the electric furnace is high and the electric furnace must be made large in size. Furthermore, there is a problem that it is difficult to provide such an electric furnace simultaneously with a porous preform fabrication apparatus.